Inspired by Robert Louis Stevenson's novella "The Strange Case of Dr. Jekyll and Mr. Hyde" (the book Mariah puts on the shelf in the beginning).
The only time where a storyteller other than Gary or his brother Tucker told a story involving the character Sardo (it was Sardo's second appearance, and the first since his apparent doom in "The Tale of the Super Specs" - David's story referenced Gary's usual storytelling, including the character, for Gary's birthday. This may also explain the minor character named Gary who appeared in it).
The only time a Midnight Society member (Gary) was shown celebrating a birthday.
Gary's future stories about Sardo have him recommending his Popularity Potion, the item invented in David's story here.
The only time Sardo's name is mispronounced in a different way (rather than addressed with an unaccented "Sardo", Keith misreads his ad and asks for "Mr. Sourdo".)